Anal. Chem. 1984, 56, 1927-1930 (15) Qulmby, 8. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 2112-21 18. (16) Frlcke, F. L.; Rose, 0.; Caruso, J. A. Talanta 1975, 23, 317-320. (17) Runnels, J. H.; Gibson, J. H. Anal. Chem. 1987, 39, 1398-1405. (18) Robbins, W. 8.; Caruso, J. A.; Fricke, F. L. Analyst (London) 1979, 104, 35-40. (19) Barett, P.; Copeiand, T. R. “Applications of Plasma Emission Spectroscopy”, 139, Barnes, R. M., Ed.; Heyden: London, 1979. (20) Ishizuka, T.; Uwamino, Y. Alia/. Chem. 1980, 52, 125-129. (21) Lichte, F. L.; Skogerboe, R. K. Anal. Chem. 1973. 45, 399-401. (22) Beenakker, C. 1. M.; Bosman, B.; Boumans, P. W. J. M. Spectrochim. Acta, Part B 1978, 338, 373-381. (23) Layman, L. R.; Hieftle, G. M. Appl. Spectrosc. 1979, . . Bvstroff, R. I.; 33, 230-240. (24) Layman, L.; Hieftje, G. M. Anal. Chem. 1974, 4 6 , 322-323. (25) Winefordner, J. D.; Vickers, T. J. Anal. Chem. 1984, 36, 1939-1946. (26) Instrumentation Laboratories, Publication AID 91, Wiimington, MA.
1927
(27) Strutt, R. J. Proc. R . SOC.London, Ser. A 1917, 93, 254-267. (28) Goudmand, P.; Pannetier, 0.; Dessaux, 0.; Marsigny, L. C.R. Hebd. Seances Acad. Sci. 1983, 256, 422-424. (29) Pannetier, G.; Goudmand, P.; Dessaux, 0.; Tavernier, N. J. Chlm. P h y ~ 1984, . 61, 395-406. (30) Lewis, E. P. Phys. Rev. 1904, 18, 124-128. (31) Strutt, R. J. R o c . R . SOC. London, Ser. A 1911, 85, 219-229. (32) Strutt, R. J. R o c . R . SOC.London, Ser. A 1913, 88, 539-549. (33) Wright, A. N.; Winkier, C. A. “Active Nitrogen”; Academic Press: New York, 1968. (34) Starr, W. L. J. Chem. Phys. 1965, 43, 73-75.
RECEIVED for review March 1,1984. Accepted April 23,1984. This work was supported by the Office of Naval Research and by the National Science Foundation.
Use of Organic Solvents for Inductively Coupled Plasma Analyses P. Barrett* and E. Pruszkowska Perkin-Elmer Corporation, Spectroscopy Division, 901 Ethan Allen Highway, Ridgefield, Connecticut 06877
The effect of important parameters in ICP, such as the incident power, nebulizer gas flow, and diameter of Inlector tube, on bgckground and analyte emission was investigated. By use of the C line, C,, CN bands and several atom and ion lines, changes In their intensity as a function of the parameters were investigated. Optimum condltions were chosen and several oil samples were analyzed. Results together with recommended values are presented.
effects of rf power, nebulizer parameters and the nature of the solvent on the detection limits and on spectral interferences were also discussed. The purpose of our study was to examine how parameters such as an incident power, nebulizer flow, and diameter of injector tube affect analyte and background emission using organic solvents. The main objective was good plasma stability for a variety of solvents without sacrificing analytical performance. A set of optimum conditions was chosen. Several elements were determined in oil samples using the optimized conditions.
The inductively coupled plasma (ICP) has become an important and widely used technique for multielement analyses. Its application has extended to a wide variety of matrices, including organic samples. Determinations of trace metals in oils have been reported (1-5). Not only metallic elements but also nonmetals such as S and P have been determined in oils (6,7). The analysis is simple and fast because oil samples diluted with an organic solvent are introduced directly into the ICP. Various solvents, including xylene, MIBK, kerosene, chloroform, methanol and others, have been used as diluents (8,9). Several workers have reported difficulties in sustaining a stable plasma, especially when volatile solvents were used ( 3 , 4 , 8 , 9 ) .With organic solvents, the ICP has also been used as a detector after separation by liquid chromatography (10, 11) and after extraction (12). In a study by Boorn and Browner (8) the quantitative effects of 30 common organic solvents on analytical signals obtained from a low power (1.75 kW) argon ICP were studied. The tolerance of an ICP discharge for organic solvents was discussed in terms of the “limiting aspiration rate”. The limiting aspiration rate for a particular solvent was defined as an uptake rate permitting stable plasma operation with no appreciable carbon deposition on the inner torch surfaces for a period of 1h. A reasonable correlation was found between limiting aspiration rates and evaporation factors for a number of solvents. In general, the ICP has decreasing stability as evaporation rates of solvents increase. This indicates that solvent vapor loading is the major factor influencing plasma stability with organic solvent introduction. Moreover, the
EXPERIMENTAL SECTION A Perkin-Elmer Model ICP/5500 inductively coupled plasma emission spectrometer equipped with the Model 3600 data system and a PR-100 printer was used. The instrument utilizes a 27.12-MHz72.5-kW generator and automatching network. A demountable torch, a dual-tube spray chamber, and a cross-flow nebulizer were used for all experiments. The spray chamber and nebulizer/end cap assembly are made by injection molding using Ryton. A Model 056 strip-chart recorder was used for monitoring analyte peaks and plasma background. A Rabbit peristaltic pump (Rainin Instrument Co.) introduced sample solution into the nebulizer. The nebulizer argon flow was monitored by a precision flowmeter with a needle valve instead of the gas pressure gauge supplied with the instrument. Standard solutions of elements were prepared from Conostan metalloorganic standards (Ponca City, OK) diluted with appropriate solvents. A Conostan S-21 blended standard of 21 elements or single-element Conostan standards were used. Organic solvents used as diluents were reagent grade except for kerosene which was technical grade. The oil samples analyzed in this study were the following NBS Standard Reference Materials: 1634 and 1634a Trace Elements in Fuel Oil; 1084 and 1085 Wear Metals in Lubricating Oil; 1621a, 1622a, 1634a, and 1634 Sulfur in Residual and Distillate Fuel Oil.
0003-2700/84/0356-1927$01.50/0
RESULTS AND DISCUSSION A series of experiments were performed to show the effect of changing parameters on several atom and ion line intensities as well as on some carbon atom and band intensities. The investigation of carbon species allowed us to examine the efficiency of organic matrix destruction. Measurement of atom 0 1984 American Chemical Society
1028
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984 I
I 60
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Figure 1. Effect of power on emission intensities of the C line and the C2 and CN bands: (-) 359.04-nm CN; (- - - ) 193.09-nm C; (- -) 436.52-nm C,. Xylene was used as solvent.
N E B . G A S FLOW (L/rnin.)
Figure 3. Effect of nebulizer gas flow on emission intensities of the 199.36-nm C line and the 436.52-nm C2 band: (-) 199.36-nm C; (---) 436.52-nm C2. Xylene was used as solvent.
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Figure 2. Effect of power on emission intensities of some element lines: (-) 206.20-nm Zn (15.4 eV); (---) 317.93-nm Ca (13.1 eV); (---) 336.12-nm Ti (10.5 eV); ( - e - ) 334.50-nm Zn (7.8 eV); (---) 422.67-nm Ca (2.9 eV). Xylene was used as solvent.
Flgure 4. Effect of nebulizer gas flow on emission intensities of three Zn lines: (---) 213.86-nm (5.8 eV); (-) 206.20-nm (15.4 eV); (---) 334.50-nm (7.8 eV). Xylene was used as solvent.
or ion intensities helped determine optimum conditions for the best SIN ratio. All of the plots are constructed from “net” emission signals after the background was subtracted. The viewing height was 15 mm for all experiments. Figure 1 shows changes in the C line and changes in Cz and CN band intensities with increasing incident power. These bands and the line emission are present from the organic solvent, in this case xylene. Although additional C lines and Cz and CN bands were studied, Figure 1 shows only three representative lines for clarity. Others showed similar trends. The intensity of the C line increases with increasing power, the intensity of the Cz band decreases, and the CN emission signal is generally stable. An increase in power causes an increase in the amount of energy available in the plasma and promotes dissociation of organic solvent, thus reducing Cz emission and increasing C emission. The CN signals are not affected as much, because formation of CN occurs in the upper part of the plasma due to air entrainment and its residence time is short. The overall effect of increasing the incident power is to help dissociate the organic matrix. Figure 2 also shows the effect of power but on the intensity of atom and ion lines, using xylene as a solvent. These lines exhibit a wide range of excitation potentials from 2.9 eV to 15.4 eV. The intensities of lines with high excitation potential (13 eV and higher) gradually increase with power, while intensities of lines with lower potentials show little variation with power. Higher powers have little effect on improving the net signal even when transitions with high excitation potentials are examined. We then investigated the relationship between emission intensity and nebulizer gas flow. The C line and Cz band represent xylene background (Figure 3). The C line shows
a maximum intensity at about 0.5 L/min, while the Cz intensity increases with the increasing nebulizer flow. The intensity increases even more rapidly at higher flows above 0.7 L/min. With a high flow of nebulizer gas, the plasma is cooled by the increase in vapor loading, and the conditions for dissociation and ionization become less favorable. Also, the residence time of atoms and ions in plasma is shorter, because the linear velocity of nebulizer flow is greater. Therefore, with a high flow we observe that the signal from C decreases after reaching the maximum, while Czintensity starts to increase rapidly. The effect of nebulizer gas flow was next measured using xylene for three Zn lines with varying excitation potentials (Figure 4). The flow at which the highest net emission intensity is obtained was considered to be the optimum gas flow. The data for these three lines showed that for higher excitation potentials the optimum gas flow is shifted to lower values. These lines are of different intensities and the relative intensity differences between lines are not important. To investigate this observation further, we chose 15 wavelengths of several elements with a wide range of excitation potentials and repeated the experiment, using chloroform, kerosene, xylene, and water as solvents. The results as plots of optimum nebulizer flows vs. excitation potentials are shown in Figure 5. The relationship between optimum nebulizer flows and excitation energy is not a continuous function when water and kerosene are used as solvents. In this case, we obtained higher values of optimum nebulizer flow for ion lines than for atom lines with similar excitation energy. This was not observed when xylene or chloroform was used as a solvent. In water and in kerosene for lines with energy between 2.9 and 15.9 eV we got a wide range of optimum nebulizer flows. For lines with low excitation potentials the dependence of optimum
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
Table 11. Detection Limits (DL) for 213.86-nm Zn Line in Various Solvents
atom ion
lines lines
!
1.3 -
i
xylene kerosene
\
chloroform
'.
1.2 -
-'1 -
solvent
\. \. \
1.4
1.0 1.1
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DL, mg/L
solvent
DL, mg/L
0.0021 0.0032 0.11
MIBK methanol
0.0020
water
0.0030 0.0040
Injector tube 0.8 mm i.d.
Table 111. Detection Limits (DL) for 213.86-nm Zn Line in Xylene for Injector Tubes of Different Inside Diameter
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DL, mg/L
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0.0021 0.0021
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0.0028 0.0027
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Figwe 5. Correlation between excitation Potentials of lines and nebulizer flow at which maximum emission signal is obtained: (---) water; (-- -) kerosene; (-) xylene; (---) chloroform. Water, kerosene, xylene, and chloroform were used as solvents.
Table I. Optimum Nebulizer Gas Flow for 213.86-nm Zn Line vs. Evaporation Factor for Several Solvents
solvent
evaporation factor,
water kerosene xylene chloroform
opt neb gas flow:
pm3/s
L/min
13.1
0.80 0.75
18.5 321
0.60 0.35
Iniector tube 2.0 mm i.d. nebulizer flow on potential is even larger. In xylene and particularly in chloroform, optimum nebulizer flows were not as dependent on excitation potentials. Therefore, with xylene and chloroform, a nebulizer gas flow can be selected close to optimum for lines of different excitation potentials. For water and kerosene the selection of lines should be more careful, and excitation potentials of selected lines should be taken into account. The different behavior among solvents may be explained by the different evaporation factors (Table I). Compared with xylene, chloroform has a very high evaporation factor (321 pm3/s). An experiment was conducted to measure the quantity of solvent entering the discharge for both chloroform and xylene. Using the optimum nebulizer flows of 0.35 L/min for chloroform and 0.6 L/min for xylene and an 0.8 mm i.d. injector tube, we measured a volume of solvents aspirated by the nebulizer and a volume going into the drain. The results show that approximately 14% of the aspirated xylene enters the plasma while as much as 35% of chloroform is introduced. Therefore, the plasma vapor loading from chloroform is very high, so that even for atom lines with low excitation potentials, the highest intensity is obtained a t a relatively low nebulizer flow. A higher nebulizer flow (0.6 L/min) for chloroform causes a large increase in plasma loading (50% of the solvent is transported into the plasma) and thus less energy is available for analyte excitation. The evaporation factors may explain differences between solvents for a single transition but not discontinuity within some solvents. Table I shows
NEB.GAS FLOW (L/rnin.)
Figure 6. Effect of nebulizer gas flow and diameters of nebulizer tubes on emission intensity of the 213.86-nm Zn line: (---) 2.3 mm i.d.; (-) 2.0 mm i.d.; (---) 1.6 mm lad.;(---) 0.8 mm i.d.
the relationship between the evaporation factor and the nebulizer gas flow a t which the maximum signal is obtained for the Zn line in several solvents. The optimum nebulizer gas flow is lower for solvents with higher evaporation factors. Detection limits for the 213.86-nm Zn line in various solvents, measured at the optimum nebulizer gas flow for each solvent, are given in Table 11. In xylene, kerosene, MIBK, and methanol, the detection limits are quite similar. In water, the detection limit is about two times poorer than in the organic solvents. In chloroform, the detection limit is about 50 times poorer than in the other organic solvents. The sensitivity for Zn in the various solvents a t the optimum nebulizer flow rates is about the same, except for chloroform where it is much less. Chloroform has a much higher evaporation fador which increases plasma vapor loading. This may explain the lower net intensity and higher base line noise and, consequently, the much poorer detection limit. It has been reported (9) that a smaller diameter of injector tube improved the stability of the plasma while running volatile organic solvents. The pressure in the spray chamber is higher and the velocity of sample aerosol introduced into the plasma is higher, thus reducing vapor plasma loading. Figure 6 shows the effect of various diameters of injedor tubes on emission intensity and on optimum nebulizer flow. Reduction in the diameter of an injector tube shifts the maximum signal to lower values of nebulizer gas flow. It also causes a decrease in emission intensity of a line. Data were taken for the 213.86-nm Zn line, xylene was used as solvent, all injector tubes were made of alumina, and diameter of injector tubes was between 0.8 and 2.3 mm. Table 111shows detection limits for the 213.86-nm Zn line obtained with injector tubes of various diameters at the optimum nebulizer flow for each diameter, using xylene as a solvent. When organic solvents are run with smaller injector tubes, the plasma stability is visibly improved and the base line noise reduced. Therefore, in spite of lower sensitivity for smaller injector tubes and different plasma conditions such
1930
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
Table IV. ICP Operating Conditions
argon flow
plasma auxiliary nebulizer incident reflected
rf power
sample uptake rate viewing height
Table VI. Determination of Wear Metals in Oils
16 L/min 1.6 L/min 0.6 L/min 1.75 kW e10 w 1 mL/min 15 mm
Table V. Determination of Trace Metals in Fuel Oil
sample
element
NBS 1634a
Zn Ni Fe V
NBS 1634
Ca Ni Fe
v
NBS results, rglg
our results, cLg/g
2.7 f 0.2 29 f 1 -31 56 f 2 16 36 f 4 13.5 f 1 320 f 15
2.8 f 0.3 27 f 2 26 f 4 57 f 2 16 f 1 35 f 2 20 f 2 318 f 10
-
as changes in loading and velocity of sample aerosol, detection limits with injectors in the range 0.7-2.0 mm i.d. are very similar. Our experience indicates that it is easier to obtain a symmetrical plasma and a stable discharge when an injector tube of 1.5 mm or less is used for kerosene, xylene, and chloroform and 1.0 mm or less for more volatile solvents such as methanol and MIBK. Using the conditions in Table IV and a 1.2-mm injector tube, we determined several elements in NBS fuel oil samples, 1634a and 1634. The samples were diluted 50 times (1 g of a sample to 50 g of solvent) in xylene and aspirated into the plasma. Our results are similar to the NBS recommended values, except for Fe in sample 1634 (Table V). The results shown in this and subsequent tables are the mean of ten replicates. The confidence intervals expressed are the standard deviation of the ten measurements. We observed that solvents such as kerosene or MIBK are not always suitable diluents for heavy fuel oil samples which sometimes do not dissolve completely. When the NBS wear metals in oil 1084 and 1085 were analyzed, the results obtained for several elements in xylene, kerosene, and MIBK (Table VI) were in good agreement with recommended NJ3S values. The samples were diluted 50 times (1 g of a sample to 50 g of solvent). The oil used as base oil in these standards was much lighter than that used in NBS fuel oil samples. Sulfur was also determined in four NBS oil samples, 1621a, 1622a, 1634a, and 1634. The samples were diluted 100 times in xylene (0.5 g of sample to 50 g of solvent). Three sulfur wavelengths at 180.76, 182.03, and 182.62 nm were used. An argon purge was used for the determination, because there are strong oxygen absorption bands below 190 nm. Our resulta (Table VII) are in good agreement with recommended values.
CONCLUSIONS Factors such as the excitation potential of a line, diameter of an injector tube, and choice of organic solvent have a great influence on the value of a nebulizer flow at which the highest net intensity for a particular line occurs. The choice of an injector tube size depends on which solvent is used, and use of a particular solvent depends on the type of sample to be analyzed. Selection of lines depends on their sensitivity, on background around a particular line, and on the analyst’s
element
NBS results, wg/g
xylene
our results, pg/g kerosene MIBK
Sample NBS 1084 A1 Cr cu Fe Mo Ni Pb
Mg Ag Ti
98 f 2 100 f 3 98 f 4 100 f 3 97 f 5 101 f 4 101 f 4 98 f 4 ,102 99 f 5
-
100 f 3 102 f 5 99f4 98f4 97f3 98f4 96f4 97f2 97f1 101 f 7
92 f 5 100 f 1 96f3 99f2 94f3 93f3 97f2 96f2 94&2 98 f 2
103 f 7 101 f 3 101 f 3 99 f 2 99 is 4 101 f 5 100 f 3 92 f 2 96 f 5 102 f 2
Sample NBS 1085 A1
Cr cu Fe Mo Ni ?b
Mg Ai?
-
296 f 4 298 f 5 295 f 10 300 f 4 292 f 11 303 f 7 305 a 297 f 3 296
*
297 f 7 294 f 4 299 f 5 305 f 10 292 f 4 288 f 7 301 f 6 302 f 10 293 f 5
303 f 7 309 f 6 302 f 6 302 f 5 298 f 7 303 f 5 301 f 6 295 f 8 303 f 7
309 f 8 304 f 3 304 f 7 303 f 5 290 f 10 308 f 5 299 f 10 304 f 8 305 f 5
Table VII. Determination of Sulfur in Oils
sample NBS No.
NBS results, %
our results, %
1621a 1622a 1634a 1634
0.94 f 0.01 1.98 f 0.04 2.85 f 0.05 2.14 f 0.02
0.94 f 0.02 2.01 f 0.02 2.91 f 0.02 2.15 f 0.02
experience. When lines with similar excitation potential are chosen, it is easier to find a common nebulizer flow for all of them. It was observed that detection limits are not very sensitive to changes in experimental conditions. However, careful choice of all parameters can help the analyst obtain satisfactory results more easily. After examining conditions for running organic solvents, we chose a set of optimum parameters. Several NBS oil samples were analyzed and results were in good agreement with recommended values. Generally, this study proved that use of organic solvents in ICP is not difficult and results of the analysis of oil samples are accurate and reliable. Registry No. Zn, 7440-66-6;V, 7440-62-2; Ca, 7440-70-2;Ni, 7440-02-0; Fe, 7439-89-6; Al, 7429-90-5;Cr, 7440-47-3;Cu, 744050-8; Mo, 7439-98-7;Pb, 7439-92-1;Mg,7439-95-4;Ag, 7440-22-4; Ti, 7440-32-6; S, 7704-34-9; MIBK, 108-10-1;xylene, 1330-20-7; chloroform, 67-66-3; methanol, 67-56-1; water, 7732-18-5.
LITERATURE CITED (1) Fassel, V. A.; Peterson, C. A.; Abercrombie, F. N.; Kniseley, R. I . Anal. Chem. 1076, 48, 516. (2) Merryfield, R. N.; Loyd, R. C. Anal. Chem. 1979, 57, 1965. (3) Varnes, A. W.; Andrewe, T. E. Jarrell-Ash Plasma News/. 1078, f ( l ) , 12. (4) Smith, J. C. Jarfell-Ash Plasma News/. 1070, 2 (3), 4. (5) Palmer, J. M.; Rush, M. W. Analyst (London) 1982, 107, 994. (6) Ward, A. F.; Marciello, L. Jarrell-Ash Plasma News/. 1070, 2(2), 9. (7) Wallace, G. F.; Edlger, R. D. At. Spectrosc. 1981, 2 , 169. (8) Boorn, A. W.; Browner, R. F. Anal. Chem. 1982, 5 4 , 1402. (9) Noblle, A., Jr.; Shuler, R. G.; Smlth J. E., Jr. At. Spectrosc. 1082, 3, 73. (10) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1081, 53, 1223. (11) Hausier, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1227. (12) Motoka, J. M.; Mosler, E. L.; Sutley, S. J.; VIets, J. G. Appl. Spectrosc. 1970, 33, 456.
RECEIVED for review February 10,1984. Accepted May 1,1984.
